The present invention relates to a method for piecing a yarn at a rotor spinning machine and to a rotor spinning machine for carrying out such method.
With increasing demands on the yarn production process, ever higher demands are also made on the production of piecers. The process of forming piecers after yarn interruptions, the piecing, is carried out at the individual spinning stations of the open-end rotor spinning machines, generally by a piecing unit travelling along the spinning machine, the so-called piecing carriage. The piecing process is controlled by means of a piecing program.
The quality of piecers with regard to their visual appearance and strength is decisively influenced by an optimal parameterisation of the piecing program. The very complex process for determining the optimal piecing parameters until now has had to be carried out after every batch change and every change of spinning parameters, such as, for example, a change in the draft, the twist factor, the rotor speed and the like. An adequately good adjustment can often only be found after hours even with experienced users. This task is made more difficult when spinning fine yarns with high yarn counts. In the case of small yarn diameters, for example 0.2 mm with a yarn count Nm 50, it is no longer possible without a mechanical visual display of the yarn diameter for the user to visually detect the fluctuations occurring in the 100th of a millimeter range.
An important reason for the outlay for optimisation is due to the circumstance that during piecing, the fiber flow is only available in a delayed manner and not 100%. This is to be attributed to the procedure in principle during piecing.
Thus, after a process triggering the piecing, such as, for example, a yarn break or a bobbin change, the yarn band feed is switched off. The trailing opening roller, however, still releases fibers from the fiber tuft. In order to achieve the same conditions and therefore a prefeed quantity which is as far as possible the same, before each piecing process, the fiber tuft is evened out. Up to the piecing, fibers continue to be combed from the fiber tuft, so the fiber tuft is shortened.
The prefeed to form a fiber ring takes place for a predetermined time and is then switched off. In this case, the quantity of fed-in fibers as well as the duration of the prefeed can also be controlled by the adjustment of the feed speed. The process of piecing begins with the rotor start. On reaching a preadjusted piecing rotor speed, feeding in of fibers begins. In this case, a certain delay occurs in reaching the required fiber flow and possibly causes a diameter deviation after the piecer. Therefore, the fiber feed is already switched on again shortly before the start of the yarn draw-off. The draw-off speed then has a value which corresponds to the instantaneous rotor speed when maintaining the desired twist of the spun yarn. Until the operating rotor speed is reached, the draw-off speed follows the increase in the rotor speed.
Apart from the follow-up movement of the fiber flow after switching off the feed and the delayed starting up after switching on the feed, the fiber flow can also react with a delay when increasing the feed speed. This can lead to diameter fluctuations of the yarn occurring after the piecer. In order to avoid these undesired diameter fluctuations, a so-called feed addition is carried out.
When piecing, it is attempted by means of the feed addition to ensure that 100% of the required quantity of fiber is present in the rotor at every draw-off instant. The feed addition thus compensates the temporary shortfall quantity through a higher feed speed. A linear increase in the fiber flow is assumed here. The optimisation of the piecer inter alia assumes knowledge of the parameters: addition length, addition quantity and advance time of the feed, the necessary advance time of the feed being assumed to be constant with a predetermined spinning geometry. One is in a position to determine the addition length owing to suitable technical aids and the use of software for visually displaying the piecer profile.
A piecing mechanism is known from German Patent Publication DE 199 55 674 A1 set up for determining the length of the feed addition required to compensate the diameter deviation from the determined length of the diameter deviation. For this purpose, a predeterminable number of test piecers is produced without feed addition, but with reduced drafting, the number of which is determined as a function of the level of the nominal drafting.
German Patent Publication DE 199 55 674 A1 proceeds from a prior art, in which the yarn diameter in the region of the piecer is also evaluated. However, a feed addition is worked with there from the start in order to obtain, as far as possible from the beginning, piecers which can at least be used. An empirical value is used as the starting point for the addition, and is based on the average staple length. In the case of staple length distributions of natural fibers, which correspondingly fluctuate this leads in the first place to a relatively high degree of imprecision. A longer optimisation phase follows this first piecer, in which additional influences, such as the opening roller clothing, opening roller speed, rotor run-up time etc. are to be compensated and this makes the empirical determination relatively protracted. Furthermore, the result of this optimisation is only to some extent satisfactory with a very high outlay.
In contrast, German Patent Publication DE 199 55 674 A1 provides an algorithm, by means of which the length of the thin point is determined with the aid of test piecers which are produced without feed addition. So that these test piecers can be produced at all with a definite thin point at the end of the piecer, the draft is reduced for these test spinners to obtain a pieceable yarn end. This draft reduction has again to be calculated by a suitable algorithm to obtain the actual values for the thin point.
The basic information of German Patent Publication DE 199 55 674 A1 is that exclusively the length of the thin point is to be used to determine the feed addition. For this purpose, an average test piecer is calculated from a large number of individual test piecers and the increase in the yarn thickness is represented by a straight line, the point of intersection of which with the horizontal representing the normal yarn thickness is to embody the end of the thin point. The distance between the beginning of the piecer and this point of intersection is then defined as the addition length and the addition is then determined. This solution is clearly an improvement compared to the previously characterised prior art but is in need of improvement with regard to approaching the optimum of the piecer. The necessary addition level to determine the feed addition then has to be determined empirically however, which is liable to undesired imprecisions.
Because of the imprecisions occurring, the result of these measurements cannot, however, be transferred to other machines provided to process the same fiber band material and to adjust the piecing parameters.
Alternatively, the determination of the fiber flow behaviour may take place under laboratory conditions by means of video recordings of the fiber flow in the fiber guide channel. The high technical outlay does not allow this method to be used at every machine. Furthermore, the determination of the fiber flow behaviour has to be carried out again at each change of the fiber band.
Moreover, the actual effects of the fiber flow behaviour on the yarn are not detected with the two methods as the fiber flow cannot be determined at the site of the yarn formation because the interior of the rotor is not accessible for measurement purposes during operation. In addition, the two methods disregard the doubling back taking place in the rotor.